Many neurological diseases are characterized by undesirable neural activity resulting in severe symptoms. Such diseases include spasticity, movement disorders, and chronic pain of peripheral origin. A localized, reversible, electrical nerve conduction block would be an attractive way of addressing these conditions.
High Frequency Alternating Current (HFAC) waveforms have been shown to provide a very localized, immediate, complete, and reversible conduction block for motor and sensory nerve fibers in acute animal preparations without indications of nerve damage. However, HFAC produces a transient neural activity when turned on. This effect has been termed the “onset response.” The onset response can take many seconds to diminish and cease. If an HFAC nerve block were applied to a mixed nerve, the onset response could produce a painful sensation coupled with muscle contractions. The onset response has prevented the practical use of HFAC block for spasticity control and other applications. Efforts have been made to shorten the HFAC so that it generally lasts less than two seconds. These methods include the use of large HFAC amplitudes, higher frequencies (>20 kHz), and various electrode configurations. However, the initial portion of the onset response, lasting one to two seconds, is a component of HFAC block that has not been eliminated through modification of the waveform or electrode design alone.
A second form of electric nerve block can be achieved with direct currents (DC). In addition to other manipulations, slowly ramping the DC amplitude over the course of a few seconds can produce a DC block without evoking action potentials. This allows for DC nerve block without an onset response. However, application of DC waveforms results in nerve damage due probably to the creation of free radicals at the electrode-electrolyte interface after the charge injection capacity of the interface is exhausted and the voltage across the interface leaves the water-window. The water-window is the specific voltage range for each electrode-electrolyte interface that is limited by the activation energy, or applied external voltage, necessary to produce molecular oxygen and hydrogen. An advantage of a DC block is that it can be achieved without causing an onset response by gradually ramping the current amplitude. This is an effect that has not been achieved with HFAC block waveforms.
As such, a need exists for a better method of blocking neural conduction.